LHDrad:
(old ``supergiant module'')
It uses long characteristics and is restricted to an equidistant grid and
open boundaries at all surfaces.
Note that the switch -Drhd_r01=1 has to be set during compilation
(see Sect. 3.4).

MSrad:
(``solar module'')
It uses long characteristics. The lateral boundaries have to be periodic.
Top and bottom can be closed or open.
Note that the switch -Drhd_r02=1 has to be set during compilation
(see Sect. 3.4).

SHORTrad:
(new ``supergiant module'')
It uses short characteristics and is restricted to an equidistant grid and
open boundaries at all surfaces.
Note that the switch -Drhd_r03=1 has to be set during compilation
(see Sect. 3.4).

integer n_radminiter:
Usually the stability considerations dictate a radiative time step
smaller than the hydrodynamics or tensor viscosity time step.
To remedy this situation it is possible to allow several radiation transport steps
per global time step.
Hitherto, all three radiation transport modules support this iteration.
The minimum number of iterations (radiative sub-steps) can be specified e.g. with

If less iterations are needed the time step limit for the next step is increased.
This value will in almost any case (for explicit radiation transport) be set to 1.
In the implicit case it is set to a higher value (typically 5).

integer n_raditer:
After each complete radiative time step the recommendation for the next time step
will be chosen so that n_raditer iterations will (probably) needed.
The parameter can be set e.g. with

For a simulation of a solar-type star (with comparatively long radiative time scales)
it will typically be set to 1.
For starts with shorter radiative time scales values around 10 may be considered.
All three radiation transport modules understand this parameter.

integer n_radmaxiter:
The absolute maximum number of iterations can be specified e.g. with

If more iterations are needed the computation for the current
time step is stopped and resumed with a smaller one (which means that the hydrodynamics
and the tensor viscosity step have to be done again).
Usually, n_radmaxiter will either be set to a values somewhat larger
than the recommended number of iterations (n_raditer)
or to 0 which disable the check for too many iterations completely.
This can be safely allowed in many cases and has the advantage that there is
no need to save the initial model before calling the radiation transport module,
which saves some memory.
To disable the iteration of the radiation transport sub-step set
n_radminiter=n_raditer=n_radmaxiter=1.
All three radiation transport modules understand this parameter.

character radraybase:
Using the modules LHDrad or SHORTrad the orientation of the base axis system
can be selected e.g. with

unity: (default) During all time steps and radiative sub-steps
the direction of the rays stays the same.

random: At each time step (and radiative sub-step) a new base axis system is
chosen at random

randomgroup: At each new time step a new base axis system is
chosen at random. It is kept for all radiative sub-steps.

Because typically only a relatively small number of rays is chosen per time step
(with radraystar) it is advisable to vary the directions of the rays
(by choosing radraybase=random or randomgroup) to cover
the entire sphere at least over a longer time.

character radraystar:
Using the modules LHDrad or SHORTrad
the list of ray directions (i.e. the number of rays and their coordinates)
relative to the base axis system can be specified with e.g.

x1: (N=1) one single ray along x1 axis (not enough to specify fluxes in all directions)

x2: (N=1) one single ray along x2 axis (not enough to specify fluxes in all directions)

x3: (N=1) one single ray along x3 axis (not enough to specify fluxes in all directions)

oktaeder: (N=3, default) octahedron

tetraeder: (N=4) tetrahedron

cube: (N=4)

ikosaeder: (N=6) icosahedron

dodekaeder: (N=10) dodecahedron

Several other choices are possible, which are meant for test purposes only.
Choosing one of the five Platonic solids (Ops! ``German-Greek'' names only, so far)
means that the 3 to 10 rays are equally distributed over the solid angle
(from the center to each corner of the respective solid).

integer n_radtheta:
Using the MSrad module the ray directions have to specified in a different way:
The number of ray sets in theta direction can be chosen with e.g.

integer n_radsubray:
Using the MSrad module
the number of rays per cell (with the same direction)
can be specified e.g. with

integer n_radsubray f=I4 b=4 n='KPHI: Number of rays per cell' c0=2
2

integer n_radband:
It can be specified whether the grey opacity table or the binned frequency-dependent
part of the opacity table is used during the computation.
The grey part contains only one bin. The other (possibly non-grey) contains one or more
bins depending on the table chosen.
The parameter is specified with e.g.

Values outside this range do not have much meaning.
The implicit transport does not work efficiently yet: It does not yield larger
time steps than possible with a sequence of purely explicit sub time steps.

real c_raditereps:
With activated implicit radiation transport (LHDrad module only)
the requested convergence accuracy of the iteration can be set e.g. with

This value has to be chosen carefully to get optimal performance.
Is the step size too small the convergence is safe but too slow.
A too large step size inhibits convergence and leads to a decrease in
the time step, which results in a bad performance, too.

real c_radtvisdtau:
Using the LHDrad module
the limit in delta optical depth (rho*kappa*dx) below which the
``radiative temperature viscosity'' (=temperature smoothing) is to be applied
can be set with e.g.

The introduction of this ``temperature diffusion'' is a somewhat desperate and
inelegant attempt to improve the behavior of the Greens function (hot cells should
be cooled, cool cells should be heated).
This diffusion is necessary for not well resolved models.
It is switched off with c_radtvisdtau0.0.

real c_radtvis:
Using the LHDrad module
the amount of the ``radiative temperature viscosity'' (=temperature smoothing)
can be specified e.g. with

real c_radtvis f=E15.8 b=4 n='Temperature viscosity' u=1
1.6

For well resolved models it should be
switched off (with c_radtvis0.0).
But often its use is necessary.